1. Field of Invention
The field of the currently claimed embodiments of this invention relates to apparatuses and methods for detecting fixation of at least one eye of a subject on a target.
2. Discussion of Related Art
In recent years, the birefringent properties of the Henle fibers surrounding the fovea of a person's eye have been used to identify the position of the fovea and the direction of gaze. This allows for one to check for eye alignment and strabismus, a risk factor for amblyopia. Screening techniques have been reported that are based on the birefringence signal derived from foveal circular scanning. In this approach, a signal s(t) consisting of several frequency components (f1=k1*fs, f2=k2*fs, f3=k3*fs, etc.) is produced, where each frequency is a multiple or multiple of a fraction of the scanning frequency fs. Some frequencies prevail during central fixation, while others appear at para-central fixation. The existence and the mixture of frequencies depend on the opto-mechanical design. In the simplest case, f2=2fs is produced during central fixation, while f1=fs prevails during off-central fixation. Existing instruments acquire consecutive epochs of s(t), with gaps between them, during which an (Fast Fourier Transform) FFT is performed. (See, for example, U.S. Pat. No. 6,027,216, the entire contents of which are hereby incorporated herein by reference.) A problem with this approach is that the FFT power spectrum is a global measure, i.e., it provides information on how much of f1 and f2 are represented in the whole epoch analyzed, but it does not provide information on exactly where these frequencies appear and for how long. With less-cooperative patients, important short lasting moments of central fixation (f2) may easily be hidden behind large low-frequency (f1) components. Analyzing short time intervals is desirable, but this is where the FFT becomes prone to noise and loses spectral resolution. There thus remains a need for improved apparatuses for detecting fixation of at least one eye of a subject on a target.